Why not use plain IPv6 instead?

Even with IPv6 you still might have stateful firewalls allowing only for outbound connection at both ends (e.g. a CPE a.k.a. “WiFi router”) and to establish communication you’d need to punch a hole in those firewalls.

V6 adoption has reached 46.82%[1]. So it is increasingly viable for this.

[1] https://www.google.com/intl/en/ipv6/statistics.html